I used to work in cust serv for large poker site, and I can tell you that nearly everyone is asked for ID on these sites - the main reason is online fraud related to multiple poker accounts, and funds fraud etc etc.

Usually customers used to be asked for paperwork before their very first withdrawal (of winnings), but imagine if you have $5K waiting there, but can't withdraw it until you send your papers in, then wait about till they verify it properly - this makes a lot of customers very upset indeed!

As a result, many sites save themselves the bother of waiting until you want to make a withdrawal, and ask for the ID (and even proof-of-address or ownership of card/bank account) up front...

This is better for you, as once it's over n done with, you won't have to do it later on - you'll be able to withdraw winnings etc without annoying delays related to routine fraud/security checks...